Please ignore non-existing pseudo-class on pseudo-element vendor extension

Validate the following fragment while treating vendor extensions as warnings:

::-webkit-scrollbar:vertical {
	border: 1px solid black;
}

The validator will report the ::-webkit-scrollbar pseudo-element vendor extension as a warning, as expected. However, it reports the :vertical pseudo-class as an error. Maybe that is formally correct, but from a practical point of view, I would very much prefer if vertical would be ignored because it applies to a vendor extension.

Start by reproducing the CSS fragment in the issue with vendor extensions treated as warnings, then locate the validator logic that handles pseudo-elements and pseudo-classes. Done means the vendor pseudo-element remains a warning while the attached :vertical pseudo-class no longer produces an error, with the existing validation behavior checked for regressions.